1. Field of the Invention
This invention relates to compressors, and more particularly to a scroll-type compressor configuration that is easily assembled and improves the efficiency and other performance characteristics of scroll-type compressors.
2. Description of the Related Art
A scroll-type compressor is a high efficiency compressor used in air conditioning systems, vacuum pumps, expanders, and engines. An example of a conventional scroll-type compressor configuration is illustrated in FIGS. 1 and 2. The scroll compressor comprises a hermetic casing A', a shaft 70', a fixed scroll plate 30', orbiting scroll plate 40', and upper frame 50'. Each scroll plate 30' and 40' has a spiral shaped wrap 31' and 41', respectively. These wraps interfit to form an interior space and a series of crescent shaped pockets. A pressure equalizing passage 44' is formed in the orbiting scroll plate to interconnect the interior space with back-pressure pocket 65' of air bushing 66'.
The orbiting scroll wrap 41' is rotationally displaced 180.degree. relative to the stationary scroll wrap 31'. An orbiting movement is imparted to the orbiting scroll 40' by an Oldham's coupling 80' fitted into an upper frame 50'. The Oldham's coupling 80' translates rotational movement, e.g., from a rotating shaft 70', to an orbiting movement. A typical orbiting scroll will orbit at about 3600 rpm. As the orbiting scroll 40' orbits around the stationary plate 30', line contacts created between the interfitted wraps form crescent shaped pockets which begin to move radially inwards towards the center of the plates. As the crescent shaped pockets move radially inwards they reduce in volume, and therefore compress the fluid contained within the pockets. A discharge port at the center of one of the plates receives high pressure from the crescent shaped pockets when they terminate at the center. By this process, low pressure fluid is introduced at the exterior perimeter of the plates and is encased within the crescent shaped pockets as the pockets begin to form. As the pockets move inwardly, the fluid pressure increases until the fluid is discharged through the discharge port.
The scroll-type compressor has many advantages over other compressors, such as reciprocating compressors. First, the continuous movement of the scroll-type compressor does not require recompression or re-expansion. Second, the continuous and smooth operation of the scroll-type compressor eliminates problems associated with the reciprocating movement of other compressors (e.g., metal fatigue is reduced), and produces about one tenth of the torque. Third, the crescent shaped pockets are paired and offset at 180.degree. thereby reducing non-symmetrical pressures and the vibrations and noise attendant thereto. Finally, because of their efficiency, scroll-type compressors may be smaller and lighter, and require fewer parts, resulting in lower manufacturing costs.
One of the most important concerns in scroll-type compressor efficiency is the tendency of the crescent shaped pockets to leak. Leakage can occur either though the vertical line contacts formed at the orbiting and stationary scroll plate interface at the front or back end of each pocket, or at the horizontal seals formed at the tips of a wrap 36'a and 46'a and the flat surface of the opposing scroll plate 46'b and 36'b. Most fluid pressure loss is through the horizontal seals.
Therefore, efforts have focused on minimizing fluid leakage past the tips of the wraps. One way of doing so is to minimize the clearance between scroll tips and the opposing plates. However, increasing the contact pressure on the scroll plate tips will cause premature wearing of the wrap tips and decrease the service life of the scroll plate.
The opposite problem is created by the pressure increase within the interior space which tends to produce an axial force separating the scroll plates. To counteract this separating axial force, air bushings 66 have been used. These air bushings 66' have back-pressure pockets 65' which are interconnected with the interior space through pressure equalizing passages 44'. Therefore, as the pressure in the interior space increases, the counteracting pressure in the back-pressure pocket will increase accordingly, thereby improving the efficiency of the compressor. An example of a conventional scroll-type compressor having this configuration is described in U.S. Pat. No. 4,557,675 to Murayama et al.
Another conventional configuration uses a back-pressure pocket located between the "fixed" scroll plate and a partition between the high pressure outlet region of the compressor and the low pressure inlet region. In this type of configuration, the "fixed" scroll plate is actually permitted to displace axially in response to the axial pressures created by the back-pressure pocket and the pressure within the crescent-shaped pockets.
These conventional configurations possess certain drawbacks that render their manufacture difficult. Moreover, these configurations operate at less than maximum efficiency due to problems encountered during the compressor's assembly or problems that are an unavoidable consequence of the compressor design.
No matter how efficient a compressor design is in theory, its individual parts must be assembled prior to use. The more complex the design, the more likely it is that parts may be damaged or misaligned during assembly. Thus, simplicity of assembly plays an important role in reducing the costs and maintaining system integrity of compressors. Reducing the number of components and eliminating any complex assembly steps are important advances in producing an efficient and reliable scroll-type compressor.
A related problem results from the extremely low tolerances that typically are required for scroll-type compressor components. For example, in conventional configurations that permit axial movement of the fixed scroll, a stop-bolt is generally used to prevent displacement past a certain point. In order to maintain a high operating efficiency, the bolt's dimensions and threads must be very precise. The cost of machining compressor components, such as the stop-bolt, to low tolerances significantly increases the overall cost of manufacture. Moreover, assembling these components requires precise assembly techniques that are highly dependant upon the skill of the assembler. Any error or imprecision during assembly detracts from the overall efficiency of the compressor once it is in use.
In those assemblies that use bolts to rigidly fix the fixed scroll to prevent any movement, including axial displacement, problems such as mechanically or thermally induced stresses can decrease the efficiency of the compressor. These systems also maintain intimate contact between the tips and the opposing plates of the scroll plates at all times. This intimate contact requires the compressor motor to overcome high static friction and inertia during the start-up phase of the compressor operation, thereby further reducing the overall efficiency of the compressor.